1. Field of the Invention
The present invention is directed to a method and apparatus for forming tubes and, more particularly, to a method and apparatus for cold rolling small diameter, thin-walled tubes on a cold pilger rolling mill.
2. Description of the Related Art
Cold pilger rolling mills having a reciprocating roll stand typically require transporting arrangements for rotating the rolling stock, i.e. the tube, pipe or rod blank, in every roll stand by a defined angle and for advancing the rolling stock by a defined distance in the rolling direction. The forward-feed movement of the rolling stock is generally carried out by means of carriages which are moved in the longitudinal axial direction of the rolling stock and which either push the stock from behind or clamp it externally. In the latter case, the clamping device is provided with a rotary drive which rotates the rolling stock along its longitudinal axis simultaneously with the advancing movement.
In other cold pilger rolling mill constructions, the rotation and forward-feed of the rolling stock are effected by means of a stationary rotary forward-feed head having circulating chains, wherein the chain links surround the stock and transport it in the rolling direction by means of friction. The rotary forward-feed head is rotationally driven by means of a planetary gear set and accordingly causes the rolling stock to rotate simultaneously as it is moved in the rolling direction.
In most cases, it is necessary to rotate and advance the rolling stock only in those time phases when the rolling stock is released by the rolls of the roll stands, i.e when the stock is positioned in a dead center region of the roll stands. In general, this means that the forward feed and rotating movements are carried out in a stop-and-go or halting manner. Since the accelerating processes take place in the millisecond range, strict requirements are set for the driving mechanisms.
In known cold pilger rolling mills, the rolling stock is advanced exclusively by the forward-feed devices or transporting arrangements located on the feed side of the rolling mill, i.e. the rolling stock is pushed through the rolling mill. Such forward-feed transporting arrangements subject the rolling stock to compressive forces and stresses acting between the forward-feed apparatus and the shaping zone of the roll stand. As a consequence, there is a risk that the rolling stock and mandrel bar will kink or buckle especially when the joint or interstice between two successive pieces of rolling stock is located between the forward-feed transporting arrangement and the end, i.e entrance point, of the shaping zone. With thin-walled or small diameter pieces of rolling stock, a commonly observed problem is that the ends of adjacent pieces of stock become wedged into one another due to the pressure load or the ends of the rolling stock are rolled into one another in the shaping zone.